GB2112679A

GB2112679A - Machine tools

Info

Publication number: GB2112679A
Application number: GB08234697A
Authority: GB
Inventors: Michael Gist; Dennis Keen
Original assignee: AE PLC
Current assignee: AE PLC
Priority date: 1981-12-07
Filing date: 1982-12-06
Publication date: 1983-07-27
Also published as: IT8224639A0; DE3245053A1; JPS58126007A; IT8224639A1; ZA829003B; IT1153378B; FR2517583A1

Abstract

A machine tool for forming non-circular or stepped bores or surfaces comprises an elongate tool holder 12 fixed at one end and having a tool 15 mounted at the other end. The tool holder 12 and a workpiece 14 are rotated relatively to one another and, in order to change the position of the tool relatively to the workpiece, tool holder deflection means are provided for acting on the tool holder between the fixed end and the tool mounting to deflect the tool holder by a predetermined amount. The holder 12 is made of two strips 12a, 12b of different metals, and a coil 22 is energised to cause holder 12 to deflect under a magnetostrictive effect. Alternatively, an eccentric weight can be mounted on holder 12. <IMAGE>

Description

SPECIFICATION Machine tools The invention relates to machine tools and more particularly to the control of the movement of tool holders in machine tools.

A conventional machine tool has a tool holder whose movement is by movement of a cross-slide, on which the holder is mounted, by motors acting through lead screws. Such a machine tool can adequately machine rotating workpieces to have a constant circular crosssection along their length. A similar mounting for the tool holder can be used where the workpiece is stationary and the tool rotated to machine a bore in the workpiece. Once again this arrangement can adequately machine bores of constant circular cross-section along the length of the bore.

Both these arrangements have the disadvantage that, due to the inertia of the mounting, lead screws and motors, they cannot, during machining change rapidly the position of the tool and thus they cannot be used to machine steps in the machined surface of the wokpiece or to machine non-circular profiles other than at very slow speeds.

According to a first aspect of the invention, there is provided a machine tool in which a workpiece and a tool rotate relatively to one another to perform a machining operation on the workpiece, and comprising an elongate tool holder fixed at one end and having a tool mounting at the other end, and tool holder deflection means acting on the tool holder between the fixed end and the tool mounting for so deflecting the tool holder during machining as to vary the position of the tool relatively to the fixed end by a predetermined amount.

In one preferred embodiment, the tool holder may be formed by two elongate members of differing materials bonded together side-byside, with the holder deflection means comprising means for producing a magnetic field around the tool holder, the materials of the members being such that the magnetostrictive length change on the members caused by the magnetic field means produces said deflection of the tool holder.

In another preferred embodiment, the tool holder is rotatable about the length thereof and the holder deflection means comprises an eccentric weight mounted on the tool holder between the fixed end and the tool mounting for causing the tool holder to deflect while rotating and also comprises means for applying a predetermined radially inwardly directed force around the tool holder rotating in said deflected path to determine the locus described by the tool mounting.

According to a second aspect of the invention, there is provided a boring machine for machining the interior surface of a stationary workpiece and comprising an elongate flexible boring bar, drive means engaging the boring bar at one end thereof for rotating the boring bar, a tool holder at the other end of the boring bar, and means for applying a controlled force to the boring bar intermediate the ends to bend the boring bar during rotation thereof thereby changing the position of a tool mounted on the boring bar by a predetermined amount relatively to the workpiece.

The following is a more detailed description of two embodiments of the invention, by way of example, reference being made to the accompanying drawings, in which: Figure 1 is a diagrammatic side elevation of a machine tool using a magnetostrictive effect for positioning a tool holder of the machine; Figure 2 is a diagrammatic side elevation of a machine tool including a tool holder carrying an eccentric weight and using a fluid bearing for positioning the tool holder; Figure 3 is a radial cross-sectional view of the fluid bearing surrounding the tool holder of the machine tool of Fig. 2; Figure 4 is a longitudinal cross-sectional view of the fluid bearing of Fig. 3; and Figure 5 is a similar view to Fig. 3 but showing the use of hydraulic springs for the positioning of the tool holder; and Figure 6 is a similar view to Fig. 3 but showing the use of an eccentric cam for the positioning of the tool holder.

Referring first to the machine tool of Fig. 1, the machine has a headstock 10 including a chuck 11 for holding and rotating an elongate tool holder 12. A cross-slide 13 of the machine tool carries the workpiece 14. The tool holder 12 is fixed to the chuick 11 at one end and the other end carries a diamond or other suitable tool 15 in a tool mounting 16. The tool holder 12 is arranged to lie parallel to the axis of a bore in the workpiece to be machined.

The tool holder 12 is formed from two strips 1 2a, 1 2b of differing materials joined side-by-side in a plane normal to the direction in which tool movement is required. The join may be an adhesive or by welding or, as is preferred, by diffusion bonding to ensure that the two strips 1 2a, 1 2b are intimately connected to one another. The metals of the strips 1 2a, 1 2b are chosen so that one of them changes in length in a magnetic field by an amount which is different from the change of length of the other in the same magnetic field. This effect is termed magnetostriction.

Preferably, one increases in length while the other decreases in length.

Suitable pairs of metals are nickel and pure iron, nickel and an alloy of 50% iron and 50% nickel, nickel and an alloy of 50% cobalt and 50% iron, nickel and an alloy of 70% cobalt and 30% iron. All percentages are by weight. It will, of course, be appreciated that this list is not exhaustive and that other suitable pairs of materials may be used.

The tool holder 12 is surrounded by an electric coil 17 between the fixed end and the tool mounting 16. The coil 17 may be wound directly onto the tool holder 12 or may be wound on a separate former with sufficient room being allowed between the former and the tool holder 12 to allow for movement of the tool holder 12. The coil 17 is connected to a supply of electric current 18 which is capable of supplying a D.C. current or an A.C.

current whose frequency and amplitude can be varied. For example, a variable frequency oscillator 19 may be used connected to the coil 17 via a power amplifier 20.

It will be appreciated that, since only the portion of the tool holder 12 within the coil 17 displays the effects of magnetostriction, only this part need be of the selected magnetosctrictive metals. The tool-carrying end of the tool holder 12 may be of mild steel.

The tool holder 12 may be clamped to diminish or prevent unwanted vibrations such as chatter. This damping may take the form of thin steel strips placed on either side of the tool holder.

In use, the machine tool is operated as follows. The tool holder 12 is rotated in the headstock 10 and the workpiece 14 is moved on the cross-slide 13 to a datum position relatively to the tool. A predetermined current is applied to the coil 17 and the magnetostriction effect on the bi-metal part of the tool holder 12 causes the tool holder 12 to deflect in a direction normal to the plane of the join between the two metal strips 1 2a, 1 2b. The amount of deflection is proportional to the strength of the magnetic field generated by the coil 17 and thus a desired deflection can be achieved by control of the variable frequency oscillator 19. In this way a circular surface of required radius can be machined by the tool 15 on the workpiece.

It will be appreciated that the position of the tool 15 can either remain constant throughout the machining operation or it can be varied at various times during the machining operation to produce a finished workpiece of a circular cross-section which varies along the length of the workpiece. Alternatively, the position of the tool 15 may be varied during a revolution of the workpiece to produce a workpiece having a non-circular cross-section.

The variations of current required to produce such varied cross-sections may be controlled by a computer 21.

It will also be appreciated that the tool holder 12 need not be rotated. Instead, the workpiece may be rotated and, by turning the tool holder 12 through 90 , it may be arranged in a plane normal to the axis of rotation of the workpiece to act as a turning tool on an external surface of the workpiece.

Surfaces of varying cross-section may be produced as described above.

The deflection of the tool holder 12 is temperature dependant and for this reason a cooling system 22 may be provided to ensure that the temperature of the tool holder remains generally constant. Alternatively, a system such as the computer 21, may be provided for compensating for the effects of temperature variations with time.

Referring next to Figs. 2, 3, and 4, the machine tool comprises a bed 30 having a headstock 31 on which a chuck 33 is rotatable. A workpiece 34, such as a piston whose gudgeon pin bore 35 is to be machined, is mounted on a movable slide 42 at an end of the bed 30 opposite the headstock 31.

A tool holder in the form of an elongate boring bar 36 of circular cross-section (see Fig. 3) has one end held fixedly in the chuck 33 and has a tool 37 mounted in a tool holder 38 at the other end. A weight 39 is mounted eccentrically on the boring bar 36 intermediate the ends.

A fluid bearing 40 surrounds the boring bar 36 intermediate the ends and adjacent the eccentric weight 39 and is fixed on the bed 30. The fluid bearing 40 is not a tight fit around the boring bar 36, as best seen in Fig.

3, to allow for deflection of the bar 36. The fluid bearing 40 includes eight pairs of orifices 41, the pairs being equiangularly spaced around the boring bar 36 with the orifices of each pair being axially spaced as seen in Fig.

4. Each pair of orifices 41 is connected to a supply of fluid under pressure, for example air or a liquid under pressure. The supply of fluid to each pair of orifices 41 is separately con trollabie so that the pressure of fluid emerging from the orifices can either be varied by the same amount or by differing amounts. This control may be by servo-valves under the control of a computer 43.

There may be provided a feedback control system 44 for comparing the actual locus of the tool 37 with a desired locus and for adjusting the radially inward force in acccordance with the comparison.

In use the machine of Figs. 2, 3 and 4 operates as follows. The chuck 33 is rotated and so the boring bar 36 rotates about its length. Due to the presence of the eccentric weight 39, the boring bar axis is also deflected, that is to say it tends to generate a curved conical surface of revolution around the axis of rotation of the headstock 31. The maximum diameter of this cone will be at the end of the tool holder 36 carrying the tool 37 while the end of the tool holder 36 which is held by the chuck 33 is not deflected at all.

The fluid bearing 40 acts to control this rotating deflection by applying a controlled radially inwardly directed force to the boring bar 36. If the fluid pressure emerging from the orifices 41 is the same for all orifices, the tool 37 will describe a circle whose radius will depend on the fluid pressure (see Fig. 3). If, however, the pressure of fluid emerging from the orifices 41 is not equal, the tool 37 will describe a non-circular path whose shape can be controlled by controlling the supply of fluid to the orifices.

In this way, the tool 37 can be used to shape the gudgeon pin bore 35 of the piston 34 to any required cross-section along its length. The cross-section can be circular or oval or any other required shape. The fluid bearing 40 thus acts as a cam whose shape can be varied during the machining process or be kept constant during the process.

It will be appreciated that the radially inwardly directed force need not be applied by a fluid bearing. For example, it may be applied by a plurality of hydraulic springs 45 (Fig. 5) of variable dynamic stiffness and arranged at angularly spaced intervals around the tool holder. Alternatively, it may be applied by a cam surface 46 engaged by the boring bar 36 to constrain the tool holder 36 to a predetermined path (see Fig. 6). The cam surface may have a shape which is variable during the machining operation.

It will be appreciated that a wide variety of alternative means may be provided to deflect a tool mounting during machining to vary the position of the tool relatively to the held end by a predetermined amount. For example piezo-electric forces may be used or magnet and/or mechanical effects other than those described above with reference to the drawings.

Claims

1. A machine tool in which a workpiece and a tool rotate relatively to one another to perform a machining operation on the workpiece, and comprising an elongate tool holder fixed at one end and having a tool mounting at the other end, and tool holder deflection means acting on the tool holder between the fixed end and the tool mounting for so deflecting the tool holder during machining as to vary the position of the tool relatively to the fixed end by a predetermined amount.

2. A machine tool according to claim 1, wherein the tool holder is formed by two elongate members of differing materials bonded together side-by-side, with the holder deflection means comprising means for producing a magnetic field around the tool holder, the materials of the members being such that the magnetostrictive length change on the members caused by the magnetic field means produces said deflection of the tool holder.

3. A machine tool according to claim 2, wherein the magnetic field means comprise a coil arranged around the tool holder, between the fixed end and the tool mounting thereof.

4. A machine tool according to claim 2 or claim 3, wherein a cooling system is provided for maintaining substantially constant the temperature of the tool holder during machining.

5. A machine tool according to any one of claims 2 to 4, wherein a damping arrangement is provided for damping out unwanted deflections of the tool holder.

6. A machine tool according to any one of claims 2 to 5, wherein the metals of the tool holder are pure iron for one member and nickel for the other member.

7. A machine tool according to any one of claims 2 to 5, wherein the metals of the tool holder are nickel for one member and an alloy of 50% iron and 50% nickel (by weight) for the other member.

8. A machine tool according to any one of claims 2 to 5, wherein the metals of the tool holder are nickel for one member and an alloy of 50% cobalt and 50% iron (by weight) for the other member.

9. A machine tool according to any one of claims 2 to 5, wherein the metals of the tool holder are nickel for one member and an alloy of 70% cobalt and 30% iron (by weight) for the other member.

10. A machine tool according to any one of claims 1 to 9, wherein the tool holder rotates about the length thereof during machining, with the workpiece being stationary.

11. A machine tool according to any one of claims 1 to 9, wherein the tool holder is non-rotatable and the workpiece rotates during machining.

12. A machine tool according to claim 1, wherein the tool holder is rotable about the length thereof and the holder deflection means comprises an eccentric weight mounted on the tool holder between the fixed end and the tool mounting for causing the tool holder to deflect while rotating and also comprises means for applying a predetermined radially inwardly directed force or forces around the tool holder rotating in said deflected path to determine the locus described by the tool mounting.

13. A machine tool according to claim 12, wherein the tool holder is of circular crosssection, the force applying means comprising an air bearing arranged around the tool holder with a gap therebetween to accommodate the tool holder deflection, the air applying said force around the tool holder so that the tool mounting describes a required locus.

14. A machine tool according to claim 12, wherein the force applying means comprise a plurality of hydraulic springs of variable dynamic stiffness arranged at angularly spaced intervals around the tool holder for applying a force around the tool holder so that the tool mounting describes a required locus.

15. A machine tool according to claim 12, wherein the force applying means comprise a cam surface engaged by the tool holder to constrain the tool holder to a predetermined path, the shape of the cam surface being variable during rotation of the tool holder to vary the tool mounting locus.

16. A machine tool according to any one of claims 1 to 15, wherein a control system is provided for controlling the deflection applied by the holder deflection means.

17. A machine tool according to claim 16 wherein the control means control the tool holder deflection means to apply a constant deflection throughout a machining operation on the workpiece.

18 A machine tool according to claim 16 or claim 17, wherein the control means control the tool holder deflection means to apply a deflection which varies within a revolution of the relative rotation therebetween.

19. A machine tool according to any one of claims 16 to 18, wherein the control means control the tool holder deflection means to apply a deflection which varies after a predetermined number of predetermined numbers of revolutions of the relative rotation therebetween, within a machining operation.

20. A machine tool according to any one of claims 16 to 19, wherein the control system is an electronic control system.

21. A machine tool according to any one of claims 16 to 20, wherein the control means include a computer to give a required tool mounting position.

22. A boring machine for machining the interior surface of a stationary workpiece and comprising an elongate flexible boring bar, drive means engaging the boring bar at one end thereof for rotating the boring bar, a tool holder at the other end of the boring bar, and means for applying a controlled force to the boring bar intermediate the ends to bend the boring bar during rotation thereof thereby changing the position of a tool mounted on the boring bar by a predetermined amount relatively to the workpiece.

23. A machine tool substantially as hereinbefore described with reference to Fig. 1 or to Figs. 2 to 4 or to Figs 2 to 4 as modified by Figs. 5 and 6 of the accompanying drawings.